Age-dependent large-scale fabric of the mantle lithosphere as derived from surface-wave velocity anisotropy

Systematic variations of the seismic radial anisotropy xi to depths of 200-250 km in North America and Eurasia and their surroundings are related to the age of continental provinces, and typical depth dependences of xi(R) are determined. The relative radial anisotropy xi(R) in the mantle lithosphere of Phanerozoic orogenic belts is characterized by nu S-H > nu(SV), with its maximum depth of about 70 km, on the average, while beneath old shields and platforms, it exhibits a maximum deviation from ACY400 model (MONTAGNER and ANDERSON, 1989) at depths of about 100 km with nu(SV)greater than or equal to nu v(SH) signature. An interpretation of the observed seismic anisotropy by the preferred orientation of olivine crystals results in a model of the mantle lithosphere characterized by anisotropic structures plunging steeply beneath old shields and platforms, compared to less inclined anisotropies beneath Phanerozoic regions. This observation supports the idea derived from petrological and geochemical observations that a mode of continental lithosphere generation may have changed throughout earth's history.